Traditional automatic transmission controls effect powershift gear ratio changes in accordance with a preselected shift map that is a function of transmission output speed. More particularly, each gear ratio has an associated upshift and downshift shiftpoint that is set at the factory. However, the factory upshift and downshift shiftpoints represent conservative values that are chosen to inhibit hunting or shift cycling, e.g., the transmission shifting back-and-forth between neighboring gear ratio pairs. Consequently, traditional automatic transmission controls use overly conservative shiftpoints which can limit the rimpull/speed performance of the vehicle when the driveline load limits vehicle speed at or near an upshift shiftpoint.
However, even though the traditional transmission shiftpoints are set conservatively, hunting may still result. This is shown in relation to Prior Art FIG. 14, where an example set of vehicle wheel power curves are illustrated. Note that the wheel power curves correspond to a pair of neighboring transmission gear ratios. This figure is used to illustrate how a fluctuating vehicle load may cause a hunting condition, even with conservative shiftpoints. For example, a fluctuating load is shown as an finite number of load lines that from a "wedge". The wedge represents a vehicle that is traversing a grade that has an irregular slope or a varying surface. The width of the wedge corresponds to the inclination of the slope and/or the surface variability. Consequently, a hunting condition may occur if the shiftpoints (points 2 and 3) are within the wedge. Thus, not only do traditional transmission shiftpoints limit the vehicle's wheel power, but traditional transmission shiftpoints may also result in hunting under some vehicle operating conditions.
The subject invention is directed towards a method that appropriately modifies the upshift and downshift shiftpoints over time that results in optimized transmission shifting to further improve vehicle performance, while minimizing any undesirable hunting characteristics.